Measure the cabinets corner to corner and add 1-1/2" on the exposed sides for overlay (Image 1).
Before constructing the mold (form) for the concrete, build or secure a large, level and sturdy table (Image 2) on which to build the mold and pour the concrete. To minimize any deflection caused by the weight of the wet concrete, a table made up 3/4" plywood over a grid of 2” x 6’’s should be sufficient.
The countertop mold is made of 3/4" dual-sided laminate melamine (laminate veneer is on both sides). It’s relatively inexpensive and up to the task.
Use the old countertop as a template to outline the outside edges and corners on the bottom piece of melamine (Image 1).
Using a table saw, circular saw and jig saw, cut the melamine boards to the exact size of the old countertop (Image 2).
Cut 2" melamine strips for the sides and ends (four altogether) of the mold.
Screw the melamine strips and bottom piece together with 1-3/4" countersunk screws to create a box (Image 3) for the concrete mold. Smooth out any rough edges with an orbital sander.
Use fiberglass or Masonite strips to create curves for the corners (Image 4).
The mold has to be watertight to keep the water in the cement from causing discolorations, blemishes and dry areas in the countertop. To ensure that it’s watertight, all seams and joints have to be caulked with silicone. But silicone can also discolor concrete, so it's important to smooth out the silicone very neatly and evenly to form an unobtrusive edge in the seams.
Before caulking, tape all interior planes of the mold (with painter's tape) on either side of all seams and joints, leaving just enough room for a bead of caulk to be applied. The tape prevents the silicon from affecting other parts of the countertop.
Caulk all seams, edges and joints where the melamine pieces come together.
When it’s all caulked go back over the bead with a dab of denatured alcohol on a finger, wiping the silicone smooth along all edges.
After the silicone dries, carefully peal the painter’s tape, pulling it into itself to pull it off as well as any stray silicone.
Affix any items for "relief" or "inlay" effects (these are objects that will stay in place once the concrete is set/cured) with spray adhesive to the base of the form. For this project, DIY Host Paul Ryan used a small leaf (Image 5) as the relief effect.
To make sure the concrete will release from the mold easily, douse a rag with kerosene-based release oil and cover every surface of the mold completely.
Fill all screwhead holes with colored modeling clay, which keeps cement out of the screw heads and makes for easier disassembly of the mold.
Using bolt or wire cutters, cut the "hog fence" wire mesh (Image 6) for reinforcement inside the mold, leaving about 1-1/2” around all edges. You can suspend the mesh by securing screws into the outside surface of the mold to which you secure wire hooks to “hang” the mesh inside the mold in the middle of the cement. Or as shown below, just after screeding the concrete (see Step 4), place the mesh evenly into the concrete and push it down, suspending it in the middle of the concrete.
The mixing of the cement is the most challenging task in making a concrete countertop. A lot of variables -- mostly controllable -- can affect the outcome. Still, DIY experts suggest that if you are deliberate with your planning and careful in your mixing, you can create a sturdy, predictable and satisfying mixture of cement.
Determine the rough volume of the countertop -- for a 1-1/2" layer of concrete, use 15 lbs. of concrete per square foot. From the inside of the mold, measure the length and width to determine the square footage, divided by depth. A good cement recipe is:
1 part cement (type 1 or 2)
2 parts rock (3/8-inch pea gravel)
3 parts sand
Water reducer and pigment
The amount of pigment varies by weight, so use this rough standard: Pigment should constitute about 4 percent of the total weight of the cement.
Add a small amount of water into the cement mixer. Slowly add cement, more water and then add sand, pea gravel, more water, pigment (Image 1), more water and water reducer until consistency is like thick oatmeal.
The best way to tell if you have the proper consistency is to scoop up a lump in your hand, and, if it sticks together (Image 2), it's ready. Wiggle your fingers: The mixture should only "ooze" through, not drip or pour.
If you're having problems with the mixture, don't hesitate to call your local concrete store to get assistance.
It's best to have someone help when pouring the mixture into the mold.
Transfer the wet concrete from the cement mixer to a 5-gallon bucket.
Pour the concrete into the mold and distribute it evenly.
Using the magnesium float (Image 1) to spread the concrete, evenly distribute the concrete by lightly vibrating the float, an action called "puddling." Puddling helps reduce the amount of air pockets that form in the mixture. You can also use your own gloved fingers to push the concrete into the corners of your mold.
Scrape off the excess concrete by using a clean, straight-edged 2” x 4” pulled in a slow sawing motion (called “screeding”) across the top of the mold edges, sliding back and forth over the concrete (Image 2). A concrete mound will gather as you screed. Simply remove the excess when you get to the end of the form, making sure to clear it away from your feet.
After several screedings, place the hog-fence mesh directly on the surface of the concrete. Be sure to center it (Image 3), keeping it away from the mold sides, and then push it down evenly, suspending it in the middle of the concrete.
After the hog-fence reinforcement is in, screed the top surface again.
You should also vibrate the sides of the mold with palm sander (used without sand paper, Image 4). Vibrating the mold helps to release trapped air in the concrete, ridding the concrete of any air pockets.
Let it cure two to four days.
The ideal temperature for drying concrete is between 70 and 80 degrees. Don't disturb the concrete in any way, shape or form during the drying process.
Concrete is extremely vulnerable at this stage and can be easily damaged, so be very careful and thoughtful in your handling of the mold.
Strip the sides from the mold. If you use a pry bar, NEVER pry against the concrete, only against other strips of melamine or wood. Remove the screws and pry the boards away carefully.
Use an orbital sander to smooth the edges of the concrete before flipping the countertop.
Gather several friends for help flipping the countertop over. The countertop can weigh anywhere up to 100 pounds, depending on its size. Be extremely careful flipping the countertop.
After you’ve removed the top melamine piece, sand the top well with palm sander and 150-grit paper (Image 1). This sanding will uncover any holes that need to be filled in.
Clean off the dust and sand with a shop vacuum.
Fill any holes with Portland cement and water mixed with dye. Use the same ratio as the initial concrete mix so the color will match. Take a glob out and fill in the holes by spreading the mixture with your fingers (Image 2). Be careful not to fill in any "inlay" design (ivy was used for this particular project) you may have used.
Let the filler mixture dry for at least 1-1/2 hours.
Scrape the excess filler off after it dries, and sand again. Remove the dust with a shop vacuum, and wipe with a damp rag.
Remove the effect (ivy leaf for this project) you used as an inlay. A great way to make the relief stand out is to trace it lightly (Image 3) with a pencil before sealing the countertop.
If possible, when sealing, it's best to raise the countertop onto dowels to be able to seal all the sides completely.
Seal concrete with water-based sealer, rolling it on with a paint roller and a 2” paint brush. Let the sealer dry for a couple of hours.
Sand with 150-grit sandpaper again, and clean with a damp rag.
Wax the top of the cement counter with food-safe wax.
Be sure to wax your new countertop at least once a month to build a protective layer and to maintain the shine.
After the wax dries, buff the countertop with a power buffer (Image 4). You can do this buffing by hand, but a power buffer creates a more uniform finish.
Before preparing the cabinet tops to receive the countertop, place short 2” x 4”s (Image 1) across the cabinet tops. Rest the new concrete countertop on top of the 2” x 4”s.
To keep silicone from the cabinets, tape the edges of the cabinet surface where the new countertop will sit. Place a full bead of silicone along each edge (Image 2).
With several friends and colleagues working together, gently lower the countertop onto the cabinet tops. Check the overhang alignment. Check for level.
Caulk the seams between new countertop and cabinets.
Enjoy your new concrete kitchen countertop.